Conduits of this type are mainly used for the transport of media requiring thermal insulation or those thermal insulation is useful, such as, for instance, in pipelines for distant heating, coolants or mineral oil. The sliding parts or members prevent thermal bridges between the envelope and the media pipe. The envelope is normally provided with an outer protective pipe, consisting, for instance, of cement containing fibers or asbestos, of concrete, plastics, ceramics material, metal or the like. Connected to the outer protective pipe can be a central inner pipe or several such inner pipes distributed over the cross section of the other protective pipe. The cavity between the outer protective pipe and the inner pipe is filled with an elastic foam of plastics material, preferably polyurethane foam, for instance, whereby a strong envelope with a correctly positioned cavity for receiving the media pipe is formed. The envelope has particularly good thermal insulating properties and an additional thermal insulation is provided by the air gap between the envelope and the media pipe. A conduit of this type is described in the publication WO 83/01824.
The known conduit described there has the disadvantage that the installation of the sliding parts securing the air gap between the media pipe and the envelope is comparatively elaborate. Each sliding part is formed as an individual skid in the shape of an undulating band forming inner and outer supporting surfaces. Several such skids must be distributed in the proper position around the respective cross section of the media pipe and tightened there by means of a common tightening strap on the outer periphery of the media pipe. The tightening of the skids along the supporting surfaces abutting the media pipe moreover results in a certain transfer of heat via the skids.